There is recently a keen demand for a higher post-painting corrosion resistance of steel sheets applied in home electrical appliances and automotive body. For example, with a view to preventing damage to an automobile caused by salt used for melting ice and snow on the road in a cold district in winter, importance is attached to the improvement of corrosion resistance of a steel sheet for automotive outer shell, underside and closed structures, and the demand for steel sheets having an excellent post-painting corrosion resistance is increasing.
Electro-galvanized steel sheet is widely applicable also as a substrate steel sheet for painting because of many advantages such as the quality of steel sheet not deteriorated, the excellent formability and the possibility of easily applying a one-side galvanizing treatment since an excellent corrosion resistance is imparted to the steel sheet under the effect of sacrificial corrosion prevention of an electro-galvanized layer formed on the surface of the steel sheet, with applicability of a steel sheet of any desired quality, and the steel sheet is never exposed to heating to a high temperature during the galvanizing process.
In an electro-galvanized steel sheet, however, when applying paint onto the surface of the electro-galvanized layer which acts as a substrate for painting the, progress of corrosion causes occurrence of blisters in the interface between the electro-galvanized layer and the paint film. As a result, the electro-galvanized steel sheet was defective in that adhesion of a painting film to the electro-galvanized layer was seriously decreased, thus leading to a poor post-painting corrosion resistance. Occurrence of blisters on the interface between the electro-galvanized layer and the painting film is attributable to the fact that, along with the progress of corrosion, external moisture (H.sub.2 O) penetrates through the paint film into the corroded portions and is accumulated there. Since this moisture (H.sub.2 O) contains OH produced by the corrosion reaction, the corroded portions become alkaline (usually with a pH value of 10 or 11). As a result, the paint film is broken by the above-mentioned alkaline corroded portions, thus decreasing the corrosion resistance of the painted galvanized steel.
The following methods are known as those for improving corrosion resistance and paint adhesion of an electro-galvanized steel sheet:
(1) An acidic electro-galvanizing process, disclosed in Japanese Patent Publication No. 16,522/72 dated May 16, 1972, which comprises:
(a) in an acidic electro-galvanizing bath to which has been added with cobalt from 5 to 50 g/l in the form of at least one water-soluble compound of cobalt, subjecting a steel sheet to an electro-galvanizing treatment so that the electro-galvanized layer contains cobalt compounds; PA1 (b) in an acidic electro-galvanizing bath to which has been added cobalt from 0.3 to 20 g/l in the form of at least one of water-soluble compounds of cobalt and also to which has been added at least one water-soluble compound of molybdenum, tungsten or iron, subjecting a steel sheet to an electro-galvanizing treatment so that the electro-galvanized layer contains compounds of said metals. PA1 (a) a metal layer formed on the surface of a steel sheet by an electro-galvanizing treatment, which layer contains Zn, as the main constituent, and another metal, as the sub constituent, in an amount of from 0.05 to 7 wt.% relative to the total weight of the electro-galvanized layer in the form of at least one of the oxides of molybdenum, tungsten, or cobalt; PA1 (b) a metal layer formed on the surface of the steel sheet by an electro-galvanizing treatment, which layer contains Zn, as the main constituent, and another metal, as the sub constituent, in an amount of from 0.05 to 7 wt.% relative to the total weight of the electro-galvanized layer in the form of at least one of the oxides of molybdenum, tungsten, or cobalt, and also contains another metal, as a further sub constituent, in an amount of from 0.5 to 15 wt.% as inetal relative to the total weight of the electro-galvanized layer in the form of at least one of metal or compounds of iron, nickel or tin. PA1 in an acidic electro-galvanizing bath principally comprising Zn ion, and containing: PA1 at least one additive selected from the group consisting of: PA1 subjecting a steel sheet to an electro-galvanizing treatment to form a first galvanized layer on the surface thereof; and then, subjecting said electro-galvanized steel sheet having said first galvanized layer thus formed to an ordinary chromating treatment.
(2) A steel sheet serving as a substrate for coating, disclosed in Japanese Patent Publication No. 19,979/74 dated May 21, 1974, which comprises:
(3) A method for manufacturing a chromated electro-galvanized steel sheet, disclosed in Japanese Patent provisional Publication No. 83,838/76 dated July 22, 1976, which comprises:
______________________________________ (a) Cr.sup.3+ from 50 to 700 ppm, (b) Cr.sup.6+ from 50 to 500 ppm, (c) Cr.sup.3+ and Cr.sup.6+ from 50 to 700 ppm, Cr.sup.6+ being up to 500 ppm, (d) In ion from 10 to 3,000 ppm, and (e) Zr ion from 10 to 2,500 ppm, and, further containing: (f) Co ion from 50 to 10,000 ppm, ______________________________________
An electro-galvanized steel sheet manufacturing by any of the above-mentioned prior arts (1) to (3) is superior in corrosion resistance of the galvanized layer to an ordinary electro-galvanized steel sheet with a pure-zinc galvanized layer, because of the formation of an electro-galvanized layer comprising a combination of zinc and other metals. However, with regard to post-painting corrosion resistance of the above-mentioned superrior electro-galvanized steel sheet, it is impossible as in an ordinary electro-galvanized steel sheet to prevent occurrence of blisters on the interface between the electro-galvanized layer and the paint film, and hence impossible to solve the aforementioned disadvantages.
Under such circumstances, there is a strong demand for the development of an electro-galvanized steel sheet excellent in paint adherence which permits prevention of the occurrence of blisters on the interface between the electro-galvanized layer and the paint film, and is thus excellent in post-painting corrosion resistance but a process for manufacturing an electrogalvanized steel sheet provided with such properties is not as yet proposed.